1. Field of Invention
The invention is related to the test structure and its method. In particular, the invention relates to test structure and its method of the step coverage for optical waveguide production.
2. Related Art
As the Internet and networks are more widely used and multimedia more popular, there is an increasing need for broader bandwidth. Optical technology plays an extremely important role in the future of data transmission. Dense Wavelength Division Multiplexing, DWDM, is the one of the best methods for increasing bandwidth and transmission volume. It uses different wavelengths sharing the same fiber. Multiple data signals are transmitted using relative but different wavelengths through the wavelength divider on a single mode fiber. They are later separated into single light waves that normally operate on the single mode fiber. Therefore, data from different sources can be placed on a single mode fiber and increase the transmission efficiency of the optical broadband.
A complete dense wavelength division multiplexing system includes the transceivers, multiplexers/demultiplexers, optical fiber amplifiers (such as EDFA, erbium-doped fiber amplifiers), add-drop multiplexers, dispersion compensation devices, EMI filters, optical routers and other optical communication components, electrical circuitries, racks, etc. The multiplexer is an important element for separating the wavelengths. The current technology for producing multiplexers includes: light filtering, fiber grading, fiber coupling, waveguiding etc; there are commercial products available using these technologies. The optical waveguide element is capable of functioning over 64 wavelengths and is suitable for long-distance network communications. It is also highly sensitive and is not affected by electromagnetic waves; it can be used in different environment. The Planar Light wave Circuits, PLCs, are a technique using semiconductors to produce light wave circuitries on a plane for functions like: multiplexing, demultiplexing optical switching, etc.
The normal planar light wave circuits use silicon chips as base material, and deposit three layers with different rates of refraction on top. The top and bottom layers are cladding layers. The rate of refraction is n2; the middle layer is a waveguiding layer with a higher rate of refraction at n1 (n1 greater than n2). The three layers have similar fiber-structures. The front most point is a tool for entering and exiting of light waves; it guides multiple wavelengths into one optical output. Three depositing layers, limiting the light wave to the middle layer and transmitting them to the grading area, form the optical waveguide. This special grading area is a combined structure of normal light, dividing grades and focusing lens. When light waves hit the combined grading area, they reflect and divide, then move through the waveguiding layer and go back to the existing point. The originally mixed multiple wavelengths are then separated when existing. Since the planar optical waveguide construction follows a semiconductor process, it has the advantages of high stability, availability for mass production, and integratability. Controlling the refraction rate of the upper covering layer and the step coverage rate decides the quality of the final product. Before executing the final steps such as face polishing, the annealing process step is needed to finish the step coverage on the upper covering layer. However, it is difficult to determine if the step coverage of the covering layer is completed during the annealing process. If the step coverage of the upper covering layer is not ideal, once the face polishing process starts, it is too late to reverse the process. Determining if the efficiency of the step coverage of the upper covering layer is acceptable for the optical waveguide production, prior to the face polishing process, is an important quality control element.
The 1991 issue of Appl. Phys. Lett. published an article titled xe2x80x9cNew test structure to identify step coverage mechanisms in chemical vapor deposition of silicon dioxidexe2x80x9d, which included a new test structure and method of the step coverage. It uses the semi-conductor production method, constructing a cavity to test the step coverage. However, the cavity and the actual optical waveguide structure are very different. Also, hydrogen fluoride is required to form the cavity, which does not work for the optical waveguide structures with oxidation layers.
To solve the problems of the known technology, an appropriate step coverage test structure and method is required. This invention introduces a step coverage test structure and method for the optical waveguide production, which uses the silicon deep etching technique at the edge of the optical waveguide chip to create trench test structures. This allows instant etching test of the structure using etching solution and the result shows the step coverage of the upper covering layer on the optical waveguide chip.
The invention combines the production of the optical waveguide and the test structures, which co-exist on the optical waveguide chip, and uses a trench test structure to test the edge of the optical waveguide chip. This test area and the optical waveguide area have an identical upper covering layer, therefore, using the direct etching test to exam the step coverage, the step coverage of the optical waveguide can be extrapolated.
The testing method of the step coverage of the optical waveguide production described by this invention is made possible by creating a testing area during the optical waveguide production and tests the covering layer directly; the steps include: providing a substrate area with a covering layer; forming the waveguide layer on top of the oxidation layer; forming a covering layer on top of the waveguide layer using the method of micro-imaging, exposing only the test area; erasing the covering layer and the waveguide layer of the test area and exposing the substrate; creating the test structure in the shape optical waveguide in the test area; creating an optical waveguide structure on the waveguide layer and erasing the covering layer; depositing an upper covering layer completely over the test structure and the optical waveguide structure, and completing the optical waveguide structure; executing the annealing process for the whole substrate; and dropping etching solution on the test structure to test the step coverage of the upper covering layer. Multiple test structures can be constructed on the substrate outside of the optical waveguide area, for testing the step coverage after recovering procedures, if needed. For example, if it is discovered that the annealing procedure is not completed after testing, the process can be repeated, and other test structures can be used to test the step coverage until the annealing procedure is completed. Continuing the optical waveguide production with a face polishing process after ensuring the quality of the step coverage increases the production success.
Further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.